I agree that flash is preferable if not inevitable for a bunch of NAS use cases in 2022. I’d just like to draw attention to a type of issue that could be seen sometimes with SSDs, but typically not with HDDs, and that’s when more than one drive fail in the same instant. Here is a recent example from another thread:
Two drives in a mirror that fail at the exact same time. One known cause of these type of failures is loss of power at the wrong instant (on drives without PLP), though there seems to exist other, more obscure causes too.
Regardless of cause it’s a failure that pool redundancy won’t guard against. RINAB and so on so forth, but it’s relevant to mention given this thread’s discussion. Whereas HDD failures are usually due to stochastic wear and tear with a known and estimable (though not predictable) progression, SSDs have that plus some additional unknown unknowns. (at least cheaper models).